When controlling a vapour compression system, such as a refrigeration system, an air condition system or a heat pump, the supply of refrigerant to an evaporator is normally controlled in such a manner that the superheat value of refrigerant leaving the evaporator is maintained at a small, positive value. The superheat value is the temperature difference between the temperature of refrigerant leaving the evaporator and the dew point of refrigerant leaving the evaporator. Thus, a high superheat value indicates that gaseous and heated refrigerant is leaving the evaporator, and therefore the refrigeration capacity of the evaporator is not utilised optimally, and the vapour compression system is not operated in an efficient manner. On the other hand, zero superheat value indicates that the refrigerant leaving the evaporator is at the dew point. Thereby there is a risk that liquid refrigerant is leaving the evaporator. If liquid refrigerant reaches the compressor, the compressor may suffer damage, and it is therefore desirable to avoid that liquid refrigerant leaves the evaporator. Thus, a small, but positive, superheat value ensures that the vapour compression system is operated in an energy efficient manner, without risking damage to the compressor.
The supply of refrigerant to the evaporator may be controlled by controlling an opening degree of an expansion device, e.g. in the form of an expansion valve. The control signal for the expansion device may be supplied by a control arrangement, which derives the control signal on the basis of the superheat value which has been derived from suitable measured parameters.
U.S. Pat. No. 5,782,103 discloses an example of such a control arrangement. The control arrangement contains a measuring device connected to the evaporator, which device produces a measurement signal that is a measure of the superheat temperature of the refrigerant in the evaporator. The control arrangement further comprises a comparator to which the measurement signal and a desired superheat signal are arranged to be supplied. A PID controller is arranged between the comparator and the expansion valve. For rapid compensation of changes in the superheat temperature, a control signal proportional to the evaporating temperature of the refrigerant is arranged to be supplied additionally to the PID controller.
The control arrangement of U.S. Pat. No. 5,782,103 can only be used in combination with a PID control algorithm. This is a disadvantage, because in some applications another control algorithm would be more suitable.